Optically pumped atomic (OPA) devices, developed in mid-1950's, such as atomic clocks, atomic magnetometers, and atomic gyroscopes, are used in a number of scientific and advanced technology applications. The light source used for optical pumping in these OPA devices was originally a gas discharge lamp. In recent years, semiconductor lasers have become available with sufficient performance to replace the gas discharge lamps used in the conventional OPA devices. The benefits of using a laser instead of a discharge lamp include substantial reduction in size and electrical power consumption, potentially lower cost, and higher performance. However, unlike the gas discharge lamps, the wavelength of the semiconductor laser must be tuned and stabilized to the atomic resonance wavelength of interest. Miniaturization and improved reliability have led to many applications including detection of underground bombs, pipes, geophysical mapping, brain and heart imaging, and brain wave detection. In high-performance atomic devices, the stability of the laser can be of crucial importance. In order to expand the application of atomic devices, more robust, stabilized systems are needed.
Semiconductor lasers have two or more independent variables that can be adjusted to tune the wavelength of the laser. For vertical cavity surface emitting lasers (VCSEL), for example, these include: (i) the injection current which powers the laser, and (ii) the temperature of the laser diode. Each of the two variables not only affects the wavelength of the output light, but also the optical power of the output light. For high performance and high stability devices, it is necessary for both the optical power and optical wavelength to be stabilized.
In prior art, Gerginov et al., 2006 (V. Gerginov et al, Opt. Lett. 31, 1851 (2006)) describes a number of commonly used techniques to stabilize the laser wavelength and optical power. The two most commonly employed techniques for laser stabilization in OPA devices are briefly described below.
In one approach, a sensitive temperature sensor is mounted close to the laser diode to measure and stabilize the laser diode temperature. A wavelength error signal generated by an atomic vapor cell is used to control the laser injection current to keep the laser wavelength locked to the atomic resonance.
Due to the fast response of the laser to changes in injection current, the approach of generating the error signal using an atomic resonance to control the laser injection current enables locking the laser wavelength to the atomic resonance with high bandwidth. The downside of this approach is that only a limited tuning range is available to keep the laser wavelength locked with the laser injection current alone. If the diode laser temperature changes by more than one or two degrees, there may not be sufficient range to keep the laser wavelength locked to the resonance by changing the injection current. In addition, changes or fluctuations in the laser diode temperature when compensated by changing the injection current, changes the output power of the laser. Fluctuating optical power negatively affects the stability of the OPA device.
In another approach, the laser injection current is kept constant using a low-noise, stable current source. The wavelength error signal generated by an atomic vapor cell is used to control the laser diode temperature keeping the laser wavelength locked to the atomic resonance.
This approach provides a large tuning range to keep the laser locked to the atomic resonance and eliminates the need for an external temperature sensor for the laser. However, the downside of this approach is that depending on how the laser diode is mounted on a substrate, the thermal time constant of the laser diode heater can be very long, thereby decreasing the lock bandwidth. If the wavelength is ‘tightly’ locked with temperature, the lock can induce a response ‘peaking’ or ‘hump’ at the frequency of the lock bandwidth, which is often unacceptable. If the feedback loop is too loose, it can lead to low frequency drift in the output wavelength which is also undesirable. What is needed in the field is a simple and robust system and method to stabilize the laser wavelength.